1. Field of the Invention
This invention relates to complex (multi-component) functional microgels synthesized in the presence of a foreign, quantitatively predominant, particulate matter which is essentially non-reactive therewith.
Specifically, this invention relates to a process for the manufacture of novel structural aggregate pigment products from particulate matter treated with complex functional microgels.
The complex functional microgels employed, characterized by instantaneous (ultrarapid) formation kinetics, are synthesized in situ from subcolloidal reactive silico-aluminate and similar hydrosols and bivalent and multi-valent inorganic salts and/or organic, cationically active, chemical compounds with at least two reactive groups in each molecule.
2. Discussion of the Relevant Art
The scope of prior art relating to the synthesis and practical applications both of continuous and discrete gels obtained by interaction of alkali-metal silicates with sodium aluminate, soluble metal salts and/or free acids in different proportions and combinations is simply enormous, involving many thousands of patented gel products.
Most of these products differ from each other only in minor details, such as pH conditions, reagent concentrations, sequence of reagent additions, thermal regimes or particular variations in the otherwise very similar preparation procedures. Yet, these seemingly minor differences may bring about improved, or even novel, material or performance properties of the resultant end products, whose applications encompass such different fields as catalysis, pharmaceuticals, adhesives, water treatment, ion exchange, dehumidification of gases or soil conditioning.
The above can probably be comprehended best by considering that colloids are the lowest-rank systems known in nature equipped with "memory." As such, they "remember" their history in chronological detail and react accordingly in terms of their resultant properties and functional behavior. As a consequence, any intentional or accidental deviation from an established synthesis procedure or reaction conditions will inescapably cause certain differences, mostly quantitative but sometimes profoundly qualitative, in the constitution and/or functional properties of the resultant colloidal systems.
In the following, certain general fields of manufacture and utilization of various types of gels shall be discussed in some detail using examples found in the literature. While continuous single and mixed gels (mechanical blends of two or more separate gels) are inherently foreign to the principal idea of the present invention, they will be included in the general discussion for the sake of better clarity.
In-situ formation of silica or silico-aluminate gels in aqueous media for the purpose of surface coating of mineral particles has been utilized commercially for many years. For example, virtually all titanium dioxide pigments on the market are coated with a more or less dense layer of silica, or silico-aluminate, gels deposited in situ by a controlled interaction between relatively highly concentrated solutions of sodium silicate and appropriate gel-setting agents such as sulfuric or hydrochloric acids, ammonium sulfate, alum or sodium aluminate, in aqueous dispersions of the pigment. However, the surface coatings mentioned represent continuous gels which are fundamentally different from the instantaneously in-situ formed microparticulate gels (microgels) used in practicing the present invention developed by the Applicant and disclosed in co-pending patent application (Ser. No. 07/350,468; Filed May 11, 1989). Moreover, because of the slow formation kinetics and continuous structure of gels used in surface coating of titanium dioxide pigments in accordance with the present art, excessive uncontrollable cementation of individual particles into very abrasive oversized aggregates can not be avoided This particle aggregation is by far the most undesirable side effect of surface coating with continuous gels, having as a rule strongly pronounced cementing properties, since too close a proximity of individual titanium dioxide particles is most detrimental to their light-scattering efficacy. As a consequence, expensive fluid-energy comminution, and/or cumbersome ball milling, must be additionally employed.
U.S. Pat. No. 3,726,000 to Wildt, relating to the use of in-situ formed continuous alumino-silicate gels as intrinsic cements toward the preparation of composite pigments, may be considered as typical of the general prior art in this area of technology dating back for over half a century. Many other intrinsic cementing media have also been used for the same purpose, e g., sodium silicate and aluminum chloride in U.S. Pat. No. 2,176,876 to Alessandroni, aliphatic acid in U.S. Pat. No. 3,453,131 to Fadher, ethylenediamine and citric acid in U.S. Pat. No. 4,075,030 to Bundy, urea-formaldehyde in U.S. Pat. No. 4,346,178 to Economou, or silicon tetrachloride in WO 87/00544 to Jones.
Wildt's composite pigments were intended for latex-paint applications in which the resultant excessive aggregate hardness (abrasiveness), restricting or even prohibiting the use of these pigments in paper filling and coating, does not constitute a real disadvantage. Principal gel compositions and experimental procedures for in-situ formation of continuous gel cements disclosed in the patent to Wildt, are identical to those utilized commercially for many decades toward surface coating of titanium dioxide pigments mentioned previously It should be pointed out in this context that while a certain controlled level of residual particle cementation (permanent aggregation) is quite essential to synthesis of composite pigments, any permanent aggregation is totally undesirable when continuous gels are used for surface coating of titanium dioxide pigments. Unfortunately, all procedures of the present art relying on the use of continuous gels toward pigment aggregation or surface coating are lacking inherent mechanism allowing one to exercise effective control over the extent, or patterns, of the residual cementation of individual particles.
Continuous gels with particle-immobilizing and cementing functions were synthesized according to Wildt by an interaction between sodium silicate and aluminum sulfate in aqueous dispersions of particulate raw materials used for preparation of composite pigments, using relatively high concentrations of gel forming reagents. The particulate raw materials mentioned consisted of titanium dioxide as the principal optically-active ingredient, as well as coarse-particle-size, very abrasive, calcined clays (Engelhard's Satintone No.1 or No.4) and/or coarse delaminated clay as extenders. The procedure described involved several independent critical processing steps, such as heating of the reaction medium (to speed up the formation of continuous gel), alkaline pH adjustments, and long digestion periods (lasting from 30 to 60 minutes) followed by additional acidic pH adjustments. To convert the resultant reacted, dried, solidified, very hard "cake" into a particulate form suitable for use as composite pigment in latex paints, expensive fluid-energy comminution was again necessary.
The concept of immobilization of individual particles of titanium dioxide pigments and extenders (e.g., calcined clays) relative to each other is unquestionably valid and advantageous for latex paints and other related applications. However, the detrimental fractionation of pigmentary components and selective particle aggregation of TiO.sub.2 /TiO.sub.2 and extender/extender type can not be avoided in composite pigments prepared in accordance with the patent to Wildt because of the slow kinetics of the flocculation process employed. As a consequence, the resultant configurations of aggregated particles are not conducive to efficient light scattering. This has been indeed verified experimentally, composite pigments made in accordance with procedures set forth in the patent to Wildt having significantly poorer optical performance in paper filling and coating applications than analogous blends of identical titanium dioxide and calcined clay used in loose (non-aggregated) state.
Hoffmann, in U.S. Pat. No. 3,476,692, describes the preparation of a "silicomagnesium-aluminate-hydrate" gel (antacid) for use in treatment of gastric hyperacidity. In particular, the above invention pertains to a silicomagnesium-aluminate-hydrate gel certain properties of which are improved compared to those of other patented antacid products of virtually identical compositions. It should be emphasized rather strongly, however, that the terminology used routinely in colloidal-technological descriptions leaves much to be desired. For example, Hoffmann's so-called silicomagnesium-aluminate-hydrate gel is factually a mechanical blend of separately prepared silico-aluminate gel and a magnesium hydroxide gel, hence, fundamentally different from true complex gels used in practicing the present invention. In specific terms, Hoffmann's antacid gel was prepared by mixing concentrated solutions of sodium silicate and an aluminum salt under alkaline conditions for extended periods of time, e.g., 30 minutes, to form a solidified silico-aluminate cogel. This cogel was subsequently crushed and homogenized into a flowable pulp, into which a concentrated solution of magnesium sulfate was introduced gradually over a period of time lasting 3 hours. As a consequence, the in-situ precipitated magnesium hydroxide hydrate became mechanically, though intimately, dispersed within the previously fluidized pulp of the continuous silico-aluminate cogel.
Inorganic anion-exchangers and a process for their synthesis are disclosed by Duwell in U.S. Pat. No. 3,002,932. The above ion exchangers are prepared by . . . "coprecipitating mixed hydrated oxides of a pair of homolomorphic metals chosen from the group consisting of aluminum, silicon, titanium, zinc, and zirconium, the lower-valent member of said pair being present in major amount, in an aqueous medium at a pH in the range of about pH 5 to 7, drying the aqueous mixture at a temperature below 150.degree. C., and washing the dried mixture with water to remove soluble impurities therefrom." The above technology, as quoted, is based again on physical mixtures of separately formed gels rather than true complex microgels made up of intrinsically chemically bound complex macromolecules
U.S. Pat. No. 4,239,615 to Tu is typical of a vast group of patents pertaining to the manufacture and use of zeolites in catalytic cracking of hydrocarbon charges (crude oils). All such zeolite catalysts are based in principle on various modifications and extensions of continuous silico-aluminate cogels described extensively in textbook literature. It is because of the "memory" effects associated with colloidal systems, mentioned previously, that such endless varieties of related gel products with material or functional-performance differences of practically significant magnitudes can be synthesized with the aid of only two principal ingredients, namely, sodium silicate and sodium aluminate (or aluminum sulfate). As documented amply in everyday industrial experience, relatively small differences in the preparation, handling or post-treatment of such gels, the incorporation of various transient or permanent adjuvants notwithstanding, will often result in significant modification of such important product features as abrasion resistance, catalytic activity and selectivity, inhibition resistance or pore-size distribution.
In addition to using silica-alumina cogels as cracking-catalyst precursors, Tu also employed certain specific brand of anionic polyacrylamide (transient adjuvant) to modify the mechanical structure of catalyst matrix. Accordingly, after a subsequent burnout of the organic substance occluded in the latter matrix, Tu was able to obtain a more favorable pore-size distribution. As far as purely chemical functions of the anionic polyacrylamide with regard to catalyst formation are concerned, Tu cautiously offers the following hypothesis proposed also in other similar patents: "it is believed that the anionic form chemically react with the silica-alumina gel framework, rather than being physically dispersed in the gel, and thus contributes to the desired pore structure formation." A well known fact is, however, that concentrated solutions of strongly alkaline reagents used without exceptions in the synthesis of silica-alumina gels for catalyst precursors immediately coagulate virtually all organic water-soluble polymers available commercially, indicated clearly by phase separation. The overwhelming likelihood is, therefore, that the polyacrylamide adjuvant mentioned above was de facto dispersed mechanically in the gel, much in the same way though perhaps not as intimately as the in-situ formed molecularly precipitated magnesium hydroxide hydrate in Hoffmann's silico-aluminate antacid-gel matrix described in U.S. Pat. No. 3,476,692. As far as zeolites' reactivity on a molecular level is concerned, small amounts of metallic cations such as magnesium or calcium ions can be accepted indeed into the zeolite matrix, albeit by reversible ion-exchange mechanism rather than (irreversible) chemical reaction.
Dumoulin in U.S. Pat. No. 4,247,420 and Tu in U.S. Pat. No. 4,239,615, discussed previously, describe the use of auxiliary extraneous disperse phase(s) in the preparation of catalyst precursor gels. These particulate materials, used mainly as diluents or catalyst-matrix modifiers, are selected from among natural or synthetic zeolite powders and/or kaolin clays. Embedded in the continuous gel body, the relative proportion of these auxiliary particulates is restricted as a rule to less than 25% of the total mass of the catalyst.
Kaliski in U.S. Pat. No. 3,484,271 describes the formation of functional (release) coatings on moving paper webs by an in-situ interaction between consecutively applied separate solutions of organic anionic and cationic compounds with at least two functional groups in each molecule. These release coatings are made in the form of continuous, totally impervious, gel films devoid of any particulate occlusions. As a matter of fact, a particulate matter embedded in such films would destroy more or less completely these films' useful release properties.
U.S. Pat. No. 2,974,108 to Alexander discloses synthesis of stable alumino-silicate aquasols (hydrosols) with ion-exchange capacities equivalent to those of better zeolites, and also very good antisoiling properties. These aquasols are prepared with the aid of rather intricate thermal regimes and time-consuming procedures, using silicic acid (rather than straight alkali-metal silicate, or quaternary ammonium silicate, used in practicing the present invention) and sodium aluminate as the principal reagents. According to Alexander, the end product contains preferably from 5% to 20% of substantially spheroidal porous particles suspended in an aqueous medium with pH ranging between 5 and 10, the preferred diameter of aquasol particles ranging from 10 to 50 milimicron (nanometer) and particle porosity from 10% to 70%. Most importantly, however, the aquasols (hydrosols) according to Alexander are end products in themselves and chemically non reactive, whereas the hydrosols used in practicing the present invention are short-lived intermediate products characterized by a high level of chemical reactivity.
Additional comparisons with the prior art will be made hereinafter, wherever applicable It should be noted, however, that in reviewing the existing art Applicant is not aware of any references pertaining to systems that are true complex gels, with all principal molecular constituents being chemically bound within the same complex macromolecules, as differentiated from purely physical mixtures of two or more separate gels. In particular, no references have been found in the literature pertaining to complex, multicomponent, rapidly forming, micro-particulate gels (microgels) used in practicing the present invention or conditions under which these microgels ca be synthesized and/or utilized. More specifically, no references whatsoever were found in the literature regarding the use of complex microgels toward the manufacture of structural aggregate, single-component or multiple-component, pigment products for paper filling and coating, or any other application for that matter.